A high percentage of mines have a shale roof weak in tensile and shear strengths. Known types of roof bolts and support means, including expansion-shell and resin bolts do not work well in this type of material. Resultant failure and roof falls can be extensive, expensive and dangerous to miners. Attempts have been made to utilize composite or plural bolting systems simultaneously, such as, for example, expansion-shell or resin bolts with a truss system to combat the problem but difficulties are still encountered.
The tendency of a mine roof to sag between the ribs of the pillars after the coal has been mined is principally a shearing action. Shear has a tension component and a compression component. These two components must be supported or a mine roof will tend to fall. Some rock, such as limestone and sandstone, usually have sufficient tensile and shear strength to support the roof without additional support. Where shale exists in the overlying strata, however, and which is typically weak in tensile and shear strength, heretofore known and used roof supporting systems are not only tested severely but frequently have failed. It is known that shale constitutes a large percentage, for example up to approximately 50%, of roofs immediately above coal seams.
Known and heretofore used installations have not accommodated the various forces which occur or exist in mine roofs and will not accommodate both the shear within the beam and the compression component.
While the mine roof bolts taught in the above-mentioned U.S. patents and pending application have, for the most part, proven to be at least partially satisfactory in use, areas for improvement were noted in one particular form of roof bolt.
Basically, the present invention improves the form of roof bolt disclosed in the above patents and application. The forms of the disclosed invention comprise two members, one being a body member constituted by a bar of generally U-shaped cross-section having longitudinal spaced-apart side edges, with a stepped edged configuration defining a plurality of angular, longitudinally spaced cam surfaces. The edges generally were of an undulating or wavilinear configuration, and a coactive solid slide member was formed to a generally similar coactive shape having an inner surface or face of a configuration comparable generally to the wavilinear configuration or undulation of the edges of the U-shaped body member.
In use, the two members in a stacked or nested configuration were inserted into a pre-drilled hole or opening in the roof strata and then axially or linearly displaced, one with respect to the other, into an expanded position wherein the opposing outer surfaces of the members were displaced laterally, due to their configuration, into pressure-applying relation with the side wall of the opening in the roof strata.
These prior types of construction in use were capable of easy and quick installation, and easily removable after a period of time for reuse. These roof bolts firmly gripped the strata of the roof, essentially throughout their length which normally was, for practical reasons, limited to approximately 30" to 3' and effectively eliminated anchor slippage and minimized harmful effects of bolt elongation. Roof bolts of this type, in use, at least partly pre-stressed the strata or layers of materials above the roof laterally and generally provided some measure of support of compression and tension components of shear stress in the roof layers.
The present invention while directed to a somewhat similar construction or concept as described above, and particularly as disclosed in the co-pending application, incorporates improvements thereover not only in construction, but in functional overall operation of the roof bolts, and a system of bolt use, resulting in improved roof support.
The expansion sheath of the prior types had application limitations, including force components, and specific areas of use, and limited lengths.